Lithium-ion batteries are a family of rechargeable battery types in which lithium ions move from a negative electrode to the positive electrode during discharge, and back when charging. More specifically, during discharge, lithium ions Li+ carry current from the negative to the positive electrode through a non-aqueous electrolyte and separator diaphragm. The three primary functional components of the lithium-ion battery are therefore the anode, cathode and electrolyte.
Carbon or graphite has emerged as one of the most popular material for the anode. The cathode is generally one of three materials: a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate) or a spinel (such as magnesium oxide). Electrolytes may typically be selected from mixtures of organic carbonates such as ethylene carbonate or diethyl carbonate containing complexes of lithium ions.
Both the anode and cathode are therefore materials where the lithium may reversibly migrate. During insertion, lithium moves into the electrode. During extraction, lithium moves back out. The cathode half reaction may be written as:LiCoO2⇄Li1-xCoO2+xLi++xe−The anode half reaction may be written as:xLi++xe−+6C⇄LixC6 The overall reaction may be written as:Li++LiCoO2→Li2O+CoO
There is an ongoing need for higher specific capacity materials for higher energy density lithium-ion batteries. Work therefore continues on improving the carbon/graphite anode, which has a theoretical capacity of 372 mAh/g. However, the problems that are associated with finding a replacement material include identifying elements that, when participating in reversible reactions with lithium, do so in a manner that will not compromise anode performance.